Micromirror unit fabrication method and micromirror unit made by the same

ABSTRACT

A method of making a micromirror unit is provided. In accordance with the method, a micromirror unit is made from a material substrate having a multi-layer structure composed of silicon layers and at least one intermediate layer. The resulting micromirror unit includes a mirror forming base, a frame and a torsion bar. The method includes the following steps. First, a pre-torsion bar is formed by subjecting one of the silicon layers to etching. The obtained pre-torsion bar is rendered smaller in thickness than the mirror forming base and is held in contact with the intermediate layer. Then, the desired torsion bar is obtained by removing the intermediate layer contacting with the pre-torsion bar.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a micromirror unit used inoptical apparatus for the purposes of changing the direction of light.In particular, it relates to a micromirror unit of the type which isadvantageously incorporated in an optical switching apparatus (forselectively connecting one optical fiber to another to provide a lightpassage), an optical disk apparatus (for writing to or reading data froman optical disk), etc.

[0003] 2. Description of the Related Art

[0004] In recent years, optical communications techniques have beenwidely used in various fields. In optical communications, opticalsignals are transmitted through optical fibers. In general, use is madeof an optical switching device for changing the transmission path ofoptical signals from one fiber to another. To attain proper datatransmission, the operation of the switching device needs to meetseveral requirements, such as a large data-handling capacity, high-speeddata transmission, high stability, etc. In light of these requirements,it is preferable that an optical switching device incorporates amicromirror unit fabricated by a micro-machining technique. With the useof a micromirror unit, there is no need to convert an optical signal toan electrical signal in performing the switching operation between thedata input path and the data output path of the switching device. Thisfeature enables a micromirror unit to meet the above-mentionedrequirements.

[0005] An optical switching device incorporating a micromirror unitfabricated by a micro-machining technique is disclosed for example in aPCT application WO 00/20899 and a treatise titled “Fully Provisioned112×112 Micro-Mechanical Optical Crossconnect with 35.8Tb/secDemonstrated Capacity (Proc. 25th Optical Fiber Communication Conf.Baltimore. PD12(2000)).

[0006]FIG. 39 of the accompanying drawings shows the basic structure ofa typical optical switching device. The switching device 200 includes apair of micromirror arrays 201-202, an input fiber array 203 and anoutput fiber array 204. The input fiber array 203 includes apredetermined number of input fibers 203 a each of which corresponds toa micromirror unit 201 a of the micromirror array 201. Likewise, theoutput fiber array 204 includes a predetermined number of output fibers204 a each of which correspond to a micromirror unit 202 a of themicromirror array 202. A plurality of micro lenses 205 are arranged infacing relation to the ends of the respective input fibers 203 a, whilea plurality of micro lenses 206 are arranged in facing relation to theends of the respective output fibers 204 a.

[0007] In the optical data transmission, light beams L1 emitted from theinput fibers 203 a are collimated by the micro lenses 205 and strikeupon the respective micromirror units 201 a. Reflected on these units,the light beams are directed toward the second micromirror array 202.Each of the micromirror units 201 a has a mirror surface which isadjustable in orientation for causing the reflected light to be properlydirected toward a corresponding one of the micromirror units 202 a.Likewise, each of the micromirror units 202 a has a mirror surface whichis adjustable in orientation. In this arrangement, the light beam L1emitted from an input fiber 203 a can be caused to enter a selected oneof the output fibers 204 a by changing the orientation of themicromirror units 201 a and 202 a.

[0008]FIG. 40 shows the basic structure of another optical switchingdevice. The illustrated device 300 includes one micromirror array 301, astationary mirror 302 and an input/output fiber array 303. The fiberarray 303 includes a predetermined number of input fibers 303 a and apredetermined number of output fibers 303 b. The micromirror array 301includes a plurality of micromirror units 301 a disposed correspondinglyto the respective fibers 303 a, 303 b. The switching device 300 alsoincludes a plurality of micro lenses 304 each of which is arranged infacing relation to the end of a corresponding fiber 303 a or 303 b.

[0009] In the device 300 again, the respective micromirror units 301 aare adjustable in orientation to change the path of a light beam.Specifically, in the optical data transmission, a light beam L2 emittedfrom an input fiber 303 a passes through the micro lens 304 and strikesupon a micromirror unit 301 a (the “first micromirror unit” 301 a)Reflected on the first micromirror unit 301 a, the light beam L2 isdirected toward the stationary mirror 302, and reflected on the mirror302 to be directed back toward the micromirror array 301. As readilyunderstood, the returned light beam can be caused to strike upon aselected micromirror unit 301 a (the “second micromirror unit” 301 a) byadjusting the orientation of the first micromirror unit 301 a. With thesecond micromirror unit 301 a properly oriented, the reflected lightbeam L2 is caused to enter a selected one of the output fibers 303 b.

[0010] In the above-described switching devices 200 and 300, thestructure of each micromirror unit influences the overall performance ofthe switching device. For instance, the switching accuracy or switchingspeed may be altered by structural change in the switching device.Further, the control method of adjusting the inclination angle of themirror surface of a micromirror unit depends on the structure of themicromirror unit. If the control method can be simplified, it ispossible to increase the control accuracy. In addition, thesimplification of the control method will reduce the burden on acontrol/drive circuit of the device, thereby making it possible todownsize the switching device as a whole. Furthermore, opticalmonitoring and prevention of cross talk will also be simplified.

[0011]FIG. 41 shows a conventional two-axis type micromirror unit thatcan be incorporated in the above-described optical switching device 200or 300. The illustrated micromirror unit 400 includes a mirror substrate410 and a base substrate 420. The mirror substrate 410 is arranged abovethe base substrate 420 with non-illustrated spacers providedtherebetween. The mirror substrate 410 includes a mirror forming base411, an inner frame 412 and an outer frame 413. The mirror forming base411 is connected to the inner frame 412 by a pair of first torsion bars414. The inner frame 412 is connected to the outer frame 413 by a pairof second torsion bars 415. The first torsion bars 414 define a rotationaxis about which the mirror forming base 411 is rotated relative to theinner frame 412. Similarly, the second torsion bars 415 define anotherrotation axis about which the inner frame 412 (and hence the mirrorforming base 411) is rotated relative to the outer frame 413.

[0012] The lower surface of the mirror forming base 411 is provided witha pair of first conductive strips or electrodes 411 a and 411 b, whilethe upper surface of the base 411 is provided with a mirror surface (notshown) for reflecting light. The lower side of the inner frame 412 isprovided with a pair of second conductive plates or electrodes 412 a and412 b.

[0013] The base substrate 420 is provided with a pair of thirdconductive plates or electrodes 420 a and 420 b arranged in facingrelation to the first electrodes 411 a and 411 b, respectively. Inaddition, the base substrate 420 is provided with a pair of fourthconductive plates or electrodes 420 c and 420 d arranged in facingrelation to the second electrodes 412 a and 412 b, respectively. In themicromirror unit 400, the mirror forming base 411 is driven about thefirst or second torsion bars 414 or 415 by generating electrostaticforce between the above-mentioned electrodes.

[0014] With the above arrangement, the mirror forming base 411 undergoesrotation in an M3-direction (called “M3-rotation” below) about the firsttorsion bars 414, for example when the first electrode 411 a is chargedpositively and the third electrode 420 a is charged negatively. Asreadily understood, the rotation is caused by the electrostatic forcegenerated between the positive electrode 411 a and the negativeelectrode 420 a.

[0015] To cause the inner frame 412 (together with the mirror formingbase 411) to undergo rotation in an M4-direction (called “M4-rotation”below) about the second torsion bars 415, the second electrode 412 a ischarged positively, while the fourth electrode 420 c is chargednegatively. FIG. 42 shows a state in which the inner frame 412 hasundergone an M4-rotation of θ degrees. In this state, it is possible toadditionally cause the mirror forming base 411 to undergo an M3-rotationby generating an electrostatic force between the first electrode 411 aand the third electrode 420 a.

[0016] In the flat state shown in FIG. 41 and the slant state shown inFIG. 42, the orientation of the first electrodes 411 a, 411 b relativeto the third electrodes 420 a, 420 b is different. Therefore, even whenthe same voltage is applied between the first electrode 411 a and thethird electrode 420 a, the electrostatic force to be generated betweenthe two electrodes will be different in strength for the flat state(FIG. 41) and for the slant state (FIG. 42). As a result, the angle ofthe M3-rotation the mirror forming base 411 undergoes will be differentfor the two states.

[0017] Thus, when an M3-rotation of the same angle is desired for theflat state and for the slant state, a proper control is required for thevoltage to be applied between the first electrode 411 a and the thirdelectrode 420 a. Specifically, the voltage control needs to be performedin accordance with prestored data on M3-rotation angles and M4-rotationangles. Such a technique, however, tends to be troublesome due to thenecessity of the collection of the angle data and the necessity ofreference to the stored angle data in performing the voltage control. Inaddition, generally, such data is disadvantageously copious.

[0018] Due to the above drawbacks, it is difficult to increase theswitching speed of the micromirror unit 400 in which a complicatedvoltage control is required. Also, the burden on the driving circuit ofthe unit 400 tends to become unduly heavy.

[0019] Another problem the micromirror unit 400 may encounter is a“pull-in phenomenon” of the inner frame 412 and the inner frame 412 dueto the flat-electrode structure of the unit 400. Specifically, as notedabove, the base substrate 420 is provided with flat electrodes 420 a˜420d, the mirror forming base 411 is provided with flat electrodes 411 aand 411 b, and the inner frame 412 is provided with flat electrodes 412a and 412 b. With the close facing relation of these electrodes, themirror forming base 411 and/or the inner frame 412 may be excessivelydrawn toward the base substrate 420 upon application of a certainvoltage (known as “pull-in voltage”). In such an instance, it isimpossible to control the inclination angle of the mirror forming base411. This problem becomes more serious as a great inclination angle (noless than about 5 degrees) is desired.

[0020] One way to overcome the above problem is to use a “comb-teethelectrode design” for driving a micromirror unit instead of theabove-described flat electrode design. FIG. 43 shows the basic structureof a micromirror unit employing a comb-teeth electrode design. Theillustrated micromirror unit 500 includes a mirror forming base 510(whose upper or lower side is provided with a mirror surface), an innerframe 520 and an outer frame 530 (depicted fragmentally). Each of theelements 510, 520 and 530 is formed integral with comb-teeth electrodes.Specifically, the mirror forming base 510 is provided with a pair offirst comb-teeth electrodes 510 a and 510 b spaced from each otheracross the rectangular base 510. The inner frame 520 is provided with apair of second comb-teeth electrodes 520 a and 520 b extending inwardand corresponding to the first comb-teeth electrodes 510 a and 510 b,respectively. In addition, the inner frame 520 is provided with a pairof third comb-teeth electrodes 520 c and 520 d extending outward. Incorrespondence with the third comb-teeth electrodes, the outer frame 530is provided with a pair of fourth comb-teeth electrodes 530 a and 530 bextending inward.

[0021] The mirror forming base 510 is connected to the inner frame 520by a pair of first torsion bars 540. The inner frame 520 is connected tothe outer frame 530 by a pair of second torsion bars 550. The firsttorsion bars 540 define a rotation axis about which the mirror formingbase 510 is rotated relative to the inner frame 520. The second torsionbars 550 define another rotation axis about which the inner frame 520(and hence the mirror forming base 510) is rotated relative to the outerframe 530.

[0022] Referring to FIG. 44A, the first comb-teeth electrode 510 a andthe adjacent second comb-teeth electrode 520 a for example arevertically offset from each other when no driving voltage is applied.Then, upon application of the voltage, the first comb-teeth electrode510 a is drawn into the second comb-teeth electrode 520 a, as shown inFIG. 44B, thereby moving the mirror forming base 510. More specifically,referring to FIG. 43, the first comb-teeth electrode 510 a may becharged positively, while the second comb-teeth electrode 520 a may becharged negatively. As a result, the mirror forming base 510 is rotatedin an M5-direction as twisting the first torsion bars 540. On the otherhand, when the third comb-teeth electrode 520 c is charged positivelyand the fourth comb-teeth electrode 530 a is charged negatively, theinner frame 520 is rotated in an M6-direction as twisting the secondtorsion bars 550.

[0023] With the illustrated comb-teeth design, the orientation of thefirst comb-teeth electrodes 510 a, 510 b relative to the secondcomb-teeth electrodes 520 a, 520 b is maintained (supposing that novoltage is applied across the first and the second electrodes)regardless of the inclination angle of the inner frame 520 relative tothe outer frame 530. Therefore, the inclination angle of the mirrorforming base 510 relative to the inner frame 520 can be controlledeasily. Further, in accordance with the comb-teeth design, theelectrostatic force for driving the base 510 generally acts in adirection perpendicular to the rotational movement of the mirror formingbase 510. In this way, no “pull-in phenomenon” of the mirror formingbase 510 will occur in operation of the micromirror unit 500. Thus, itis possible to rotate the mirror forming base 510 through a relativelygreat angle.

[0024] In the micromirror unit 500, the comb-teeth of the electrodes arecaused to shift in position as the mirror forming base 510 and the innerframe 520 are rotating. Thus, the comb-teeth electrodes need to have agreat thickness in accordance with the inclination angle of the base 510and the frame 520. For instance, supposing that the body 511 of themirror forming base 510 has a length D of about 1 mm, when the mirrorforming base 510 is rotated relative to the inner frame 520 through 5degrees about the first torsion bars 540, one end portion 511′ of thebody 511 will be lowered by 44 μm. Therefore, the thickness T of thefirst comb-teeth electrodes 510 a, 510 b needs to be 44 μm at least.

[0025] On the other hand, in order to obtain a relatively great rotationangle with a low driving voltage, the torsion bars 540, 550 should besmall in thickness. In the conventional micromirror unit 500, however,the torsion bars 540, 550 are equal in thickness to the mirror formingbase 510 and the inner and the outer frames 520, 530. This means thatthe torsion bars 540, 550 have a fairly great thickness. According tothis design, when the thickness T of the first comb-teeth electrodes 510a, 510 b is rendered no less than 44 μm for example, the thickness ofthe torsion bars 510 a, 510 b will also be no less than 44 μm. With sucha thick torsion bar, a higher driving voltage is required to generate agreater electrostatic force between the relevant comb-teeth electrodesto twist the torsion bar appropriately. Alternatively, the width of thetorsion bar may be reduced so that the torsion bar can be twisted with asmaller force. While such a width-reduction design may work to someextent, the torsion bar may still fail to provide desiredtwist-resisting characteristics.

SUMMARY OF THE INVENTION

[0026] The present invention has been proposed under the circumstancesdescribed above. It is, therefore, an object of the present invention toprovide some solutions for overcoming or alleviating the above problems.More specifically, an object of the present invention is to provide amethod of fabricating a micromirror unit including precisely thintorsion bars. Another object of the present invention is to provide amicromirror unit produced by this method.

[0027] According to a first aspect of the present invention, there isprovided a method of making a micromirror unit from a materialsubstrate. The material substrate may have a multi-layer structure whichincludes a plurality of silicon layers and at least one intermediatelayer. The intermediate layer may be made of an insulating material. Themicromirror unit may include a mirror forming base, a frame and atorsion bar. The method of making such a unit includes the followingsteps. First, a pre-torsion bar is formed by subjecting the siliconlayers to etching. The resulting pre-torsion bar is smaller in thicknessthan the mirror forming base and held in contact with the intermediatelayer.

[0028] Then, the desired torsion bar is formed by removing theintermediate layer contacting with the pre-torsion bar. It should benoted here that the above phrase “removing the intermediate layercontacting with the pre-torsion bar” does not mean that the entirety ofthe intermediate layer is removed. To be precise, the intermediate layerhas a portion held in contact with the pre-torsion bar, and at leastthis particular portion is removed. The other portions of theintermediate layer, as will become clearer from the detailed descriptionof the preferred embodiments, may remain or may partially be removed.

[0029] With the above method, an advantageously thin torsion bar can beformed at the desired position in the resulting micromirror unit.Specifically, as viewed in a thickness direction, the intermediate layeris held between at least two silicon layers of the material substrate.Then, one of the silicon layers contacting with the intermediate layermay be subjected to etching with the use of a suitable mask until theetching reaches the intermediate layer in a manner such that onlypredetermined portions of the intermediate layer are exposed through theetched away portions of the silicon layer in accordance with the patternof the mask. A remaining portion of the silicon layer contacting withthe intermediate layer serves as a “pre-torsion bar”, that is, aprototype of the desired torsion bar. The pre-torsion bar is made tohave a thickness, width and length corresponding to those of the desiredtorsion bar. The pre-torsion bar is rendered smaller in thickness thanthe other elements such as a mirror forming base, a frame, a comb-teethelectrode, etc., which may be required in a micromirror unit.

[0030] According to the above method, use is made of an intermediatelayer upon which a pre-torsion bar is formed. The intermediate layer canbe accurately formed at a desired position in the material substrate bya known layer forming technique. Accordingly, the pre-torsion bar (andhence the torsion bar), which is to be formed on the intermediate layer,can be formed at the initially expected position in the resultingmicromirror unit. Further, in the above method, the etching to thesilicon layer is performed with the intermediate layer used as areference base. Thus, the thickness of the pre-torsion bar can beadjusted more finely than when no such intermediate layer is present inthe material substrate.

[0031] According to a second aspect of the present invention, there isprovided a method of making a micromirror unit including a mirrorforming base, a frame and a torsion bar. The method includes the stepsof: preparing a first material substrate having a multi-layer structureincluding a first silicon layer, a second silicon layer and anintermediate layer disposed between the first and the second siliconlayers, wherein the first silicon layer has a thickness corresponding tothe thickness of the torsion bar; providing a first mask pattern on thefirst silicon layer to mask a portion of the first silicon layer that isprocessed into the torsion bar; performing first etching until the firstetching reaches the intermediate layer, so that a pre-torsion barcontacting with the intermediate layer is formed; attaching a thirdsilicon layer to the first silicon layer for forming a second materialsubstrate incorporating the pre-torsion bar; subjecting the secondsilicon layer to second etching by using a second mask pattern arrangedto avoid a region corresponding to the pre-torsion bar, the secondetching being continued until the intermediate layer is exposed;subjecting the third silicon layer to third etching by using a thirdmask pattern arranged to avoid a region corresponding to the pre-torsionbar, wherein the third etching is continued until the pre-torsion bar isexposed; and subjecting the intermediate layer exposed by the secondetching to fourth etching for removing the intermediate layer contactingwith the pre-torsion bar so that a torsion bar is obtained.

[0032] Preferably, the above method may further include the step offorming a fourth mask pattern to mask the pre-torsion bar, wherein thefourth mask pattern forming step is performed after the pre-torsion baris formed in the first material substrate and before the second materialsubstrate is formed. With such an arrangement, it is possible to preventthe pre-torsion bar from being unduly etched away in the third etchingprocess.

[0033] According to a third aspect of the present invention, there isprovided a method of making a micromirror unit including a mirrorforming base, a frame and a torsion bar. The method includes the stepsof: preparing a first material substrate having a multi-layer structureincluding a first silicon layer, a second silicon layer and anintermediate layer disposed between the first and the second siliconlayers, wherein the first silicon layer has a thickness corresponding tothe thickness of the torsion bar; forming a first mask pattern to mask aregion of the first silicon layer that is to be processed into thetorsion bar; attaching a third silicon layer to the first silicon layer;subjecting the second silicon layer to first etching by using a secondmask pattern arranged to avoid a region for forming the torsion bar, thefirst etching being performed until the intermediate layer is exposed;forming a pre-torsion bar contacting with the intermediate layer bysubjecting the third silicon layer to second etching using a third maskpattern arranged to avoid a region for forming the torsion bar, thesecond etching being continued until the first mask pattern and theintermediate layer are exposed; and subjecting the intermediate layerexposed by the first etching to third etching for removing theintermediate layer contacting with the pre-torsion bar so that thetorsion bar is provided.

[0034] According to a fourth aspect of the present invention, there isprovided a method of making a micromirror unit including a mirrorforming base, a frame and a torsion bar. The method includes the stepsof: preparing a material substrate having a multi-layer structureincluding a first silicon layer, a second silicon layer and anintermediate layer disposed between the first and the second siliconlayers; forming a groove in the first silicon layer by subjecting thefirst silicon layer to first etching using a first mask pattern arrangedto avoid a region in which the torsion bar is formed, the first etchingbeing continued until the intermediate layer is exposed; coating thegroove with a silicon material; subjecting the second silicon layer tosecond etching using a second mask pattern arranged to cover a region inwhich the torsion bar is formed, the second etching being continueduntil the intermediate layer is exposed; subjecting the intermediatelayer exposed by the second etching to third etching performed on a sideof the second silicon layer, the third etching being continued until thesilicon material applied at the groove is exposed; performing fourthetching on a side of the second silicon layer to remove the siliconmaterial exposed by the third etching, so that a pre-torsion bar made ofthe silicon material and contacting with the intermediate layer isobtained; and forming the torsion bar by removing the intermediate layercontacting with the pre-torsion bar.

[0035] According to a fifth aspect of the present invention, there isprovided a method of making a micromirror unit including a mirrorforming base, a frame and a torsion bar. The method includes the stepsof: preparing a material substrate having a multi-layer structureincluding a first silicon layer, a second silicon layer and anintermediate layer disposed between the first and the second siliconlayers; forming a groove in the first silicon layer by subjecting thefirst silicon layer to first etching using a first mask pattern arrangedto avoid a region in which the torsion bar is formed, the first etchingbeing performed until the intermediate layer is exposed; coating thegroove with a silicon material; subjecting the silicon material appliedat the groove to second etching using a second mask pattern to mask aregion in which the torsion bar is formed, the second etching beingcontinued until the intermediate layer is exposed, so that a pre-torsionbar made of the silicon material and contacting with the intermediatelayer is obtained; subjecting the third silicon layer to third etchingusing a third mask pattern arranged to avoid a region corresponding tothe pre-torsion bar, the third etching being performed until theintermediate layer is exposed; and subjecting the intermediate layerexposed in the third etching to fourth etching for removing theintermediate layer contacting with the pre-torsion bar, so that thetorsion bar is obtained.

[0036] According to a sixth aspect of the present invention, there isprovided a method of making a micromirror unit including a mirrorforming base, a frame and a torsion bar The method includes the stepsof: preparing a first material substrate including a first siliconlayer; forming a groove in the first material substrate by first etchingusing a first mask pattern arranged to avoid a region in which thetorsion bar is formed; coating the groove with an intermediate layermaterial; applying a silicon material on the coating of the intermediatelayer material to fill the groove; forming a second material substratehaving a multi-layer structure which includes the first materialsubstrate, an intermediate layer covering the groove of the firstmaterial substrate and a second silicon layer contacting with theintermediate layer, the second material substrate incorporating apre-torsion bar that is made of the silicon material and held in contactwith the intermediate layer; subjecting the first silicon layer tosecond etching using a second mask pattern arranged to avoid a regioncorresponding to the pre-torsion bar, the second etching being performeduntil the intermediate layer material applied to the groove is exposed;subjecting the second silicon layer to third etching using a third maskpattern arranged to avoid a region corresponding to the pre-torsion bar,the third etching being continued until the intermediate layer isexposed; and subjecting the intermediate layer material exposed in thesecond etching and the intermediate layer exposed in the third etchingto fourth etching for removing the intermediate layer material and theintermediate layer to provide the torsion bar.

[0037] According to a seventh aspect of the present invention, there isprovided a method of making a micromirror unit including a mirrorforming base, a frame and a torsion bar. The method includes the stepsof: preparing a first material substrate including a first siliconlayer; subjecting the first material substrate to first etching using afirst mask pattern arranged to mask a region to be processed into thetorsion bar, the first etching being continued until the etching reachesa depth corresponding to a thickness of the torsion bar; preparing asecond material substrate having a multilayer structure which includesthe first material substrate, an intermediate layer held in contact withan etched surface of the first material substrate and a second siliconlayer held in contact with the intermediate layer; subjecting the secondsilicon layer to second etching using a second mask pattern arranged toavoid a region in which the torsion bar is formed, the second etchingbeing continued until the intermediate layer is exposed; subjecting thefirst silicon layer to third etching using a third mask pattern arrangedto avoid a region in which the torsion bar is formed, so that apre-torsion bar contacting with the intermediate layer is obtained; andsubjecting the intermediate layer exposed in the second etching tofourth etching for removing the intermediate layer contacting with thepre-torsion bar, so that the torsion bar is provided.

[0038] According to an eighth aspect of the present invention, there isprovided a method of making a micromirror unit including a mirrorforming base, a frame and a torsion bar. The method includes the stepsof: preparing a material substrate having a multi-layer structureincluding a first silicon layer, a second silicon layer and anintermediate layer disposed between the first and the second siliconlayer; subjecting the first silicon layer to first etching using a firstmask pattern and a second mask pattern, the first mask pattern beingarranged to mask a region in which the torsion bar is formed, the secondmask pattern being arranged to avoid a region in which the torsion baris formed, the first etching being continued until the etching reaches adepth corresponding to a thickness of the torsion bar; removing thefirst mask pattern; subjecting the first silicon layer to second etchingusing the second mask pattern, the second etching being continued untilthe intermediate layer is exposed, so that a pre-torsion bar contactingwith the intermediate layer is obtained; subjecting the second siliconlayer to third etching using a third mask pattern arranged to avoid aregion corresponding to the pre-torsion bar, the third etching beingcontinued until the intermediate layer is exposed; subjecting theintermediate layer exposed in the third etching to fourth etching forremoving the intermediate layer contacting with the pre-torsion bar toprovide the torsion bar.

[0039] According to a ninth aspect of the present invention, there isprovided a method of making a micromirror unit including a mirrorforming base, a frame and a torsion bar. The method includes the stepsof: preparing a material substrate having a multi-layer structure whichincludes a first silicon layer, a second silicon layer having athickness corresponding to the thickness of the torsion bar, a thirdsilicon layer, a first intermediate layer between the first and thesecond silicon layers, and a second intermediate layer between thesecond and the third silicon layers; subjecting the first silicon layerto first etching using a first mask pattern and a second mask pattern,the first mask pattern being arranged to mask a region in which thetorsion bar is formed, the second mask pattern being arranged to avoid aregion in which the torsion bar is formed, the first etching beingcontinued until the etching reaches a depth corresponding to a thicknessof the torsion bar; removing the first mask pattern; subjecting thefirst silicon layer to second etching using the second mask pattern, thesecond etching being continued until the first intermediate layer isexposed, so that a third mask pattern is formed at the first siliconlayer disposed on the first intermediate layer; subjecting the firstintermediate layer exposed in the second etching to third etching usingthe third mask pattern, the third etching being continued until thesecond silicon layer is exposed, so that a fourth mask pattern is formedat the first intermediate layer disposed on the second silicon layer;subjecting the second silicon layer exposed in the third etching tofourth etching using the fourth mask pattern, the fourth etching beingcontinued until the second intermediate layer is exposed, so that apre-torsion bar is formed between the first and the second intermediatelayers; subjecting the third silicon layer to fifth etching using afifth mask pattern arranged to avoid a region corresponding to thepre-torsion bar, the fifth etching being continued until the secondintermediate layer is exposed; and performing sixth etching to thesecond intermediate layer exposed in the fifth etching and to the firstintermediate layer on the pre-torsion bar, so that the first and thesecond intermediate layers contacting with the pre-torsion bar areremoved to provide the torsion bar.

[0040] In the above second etching, a comb-teeth electrode may be formedat one of the mirror forming base and the frame. Alternatively, use ismade of an etching process other than the second etching for forming acomb-teeth electrode at the mirror forming base and/or the frame.

[0041] According to a tenth aspect of the present invention, there isprovided a method of making a micromirror unit including a mirrorforming base, frames and a torsion bar. The method includes the stepsof: preparing a material substrate having a multi-layer structure whichincludes a first silicon layer, a second silicon layer having athickness corresponding to a thickness of the torsion bar, a thirdsilicon layer, a first intermediate layer between the first and thesecond silicon layers, and a second intermediate layer between thesecond and the third silicon layers; subjecting the first silicon layerto first etching using a first mask pattern arranged to avoid a regionin which the torsion bar is formed, the first etching being continueduntil the first intermediate layer is exposed; forming a second maskpattern on the first intermediate layer exposed in the first etching,the second mask pattern being arranged to mask a region in which thetorsion bar is formed; subjecting the first intermediate layer to secondetching using the second mask pattern, the second etching beingcontinued until the second silicon layer is exposed; removing the firstmask pattern; subjecting the second silicon layer to third etching usingthe first intermediate layer exposed by the removal of the first maskpattern, the third etching being continued until the second intermediatelayer is exposed, so that a pre-torsion bar contacting with the firstand the second intermediate layers is obtained; subjecting the thirdsilicon layer to fourth etching using a third mask pattern arranged toavoid a region corresponding to the pre-torsion bar, the fourth etchingbeing continued until the second intermediate layer is exposed; andperforming fifth etching to the second intermediate layer exposed in thefourth etching and to the first intermediate layer on the pre-torsionbar, so that the first and the second intermediate layers contactingwith the pre-torsion bar are removed to provide the torsion bar.

[0042] In the above methods, the etching performed to each silicon layermay be inductively coupled plasma etching. With this, it is possible toperform highly anisotropic etching.

[0043] The frames may include a first frame and a second frame connectedto the first frame by a torsion bar.

[0044] According to an eleventh aspect of the present invention, thereis provided a micromirror unit which includes: a mirror forming base;frames having a multilayer structure including a plurality of siliconlayers and at least one intermediate layer; and a torsion bar which issmaller in thickness than the mirror forming base, the torsion bardefining an axis about which the mirror forming base is rotated relativeto the frames, the torsion bar having an end connected to a portion ofthe silicon layers that is held in contact with the intermediate layer.

[0045] The above frames may include a first frame and a second frameeach of which includes a silicon layer and an intermediate layer held incontact with the silicon layer, a torsion bar having one end connectedto the silicon layer of the first frame and another end connected to thesilicon layer of the second frame.

[0046] The frames may include two intermediate layers between which onlyone of the silicon layers is disposed, wherein the torsion bar isconnected to said one of the silicon layers.

[0047] In a preferred embodiment, the frames of a micromirror unit mayinclude first and second frames each of which includes two intermediatelayers between which only one silicon layer is disposed, wherein thetorsion bar has one end connected to said one silicon layer of the firstframe and another end connected to said one silicon layer of the secondframe.

[0048] The mirror forming base of a micromirror unit may be providedwith a first comb-teeth electrode, while the frames may be provided witha second comb-teeth electrode cooperating with the first comb-teethelectrode to move the mirror forming base. When the frames include afirst frame and a second frame, the first frame may be provided with athird comb-teeth electrode, while the second frame may be provided witha fourth comb-teeth electrode cooperating with the third comb-teethelectrode to move the first frame and the mirror forming base.

[0049] Other features and advantages of the present invention willbecome apparent from the detailed description given below with referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050]FIG. 1A is a top view showing a micromirror unit according to afirst embodiment of the present invention;

[0051]FIG. 1B is a bottom view showing the micromirror unit of FIG. 1A;

[0052]FIGS. 2A, 2B and 2C are sectional views taken along lines A-A, B-Band C-C, respectively, in FIGS. 1;

[0053] FIGS. 3A-3D, 4A-4E, 5A-5D, 6A-6D and 7A-7D are sectional viewsillustrating a micromirror unit fabrication method according to a secondembodiment of the present invention;

[0054] FIGS. 8A-8E, 9A-9E, 10A-10D, 11A-11D and 12A-12D are sectionalviews illustrating a micromirror unit fabrication method according to athird embodiment of the present invention;

[0055] FIGS. 13A-13E, 14A-14E, 15A-15D, 16A-16D and 17A-17D aresectional views illustrating a micromirror unit fabrication methodaccording to a fourth embodiment of the present invention;

[0056] FIGS. 18A-18D and 19A-19D are sectional views illustrating amicromirror unit fabrication method according to a fifth embodiment ofthe present invention;

[0057] FIGS. 20A-20D and 21A-21D are sectional views illustrating amicromirror unit fabrication method according to a sixth embodiment ofthe present invention;

[0058] FIGS. 22A-22E and 23A-23D are sectional views illustrating amicromirror unit fabrication method according to a seventh embodiment ofthe present invention;

[0059] FIGS. 24A-24D and 25A-25C are sectional views illustrating amicromirror unit fabrication method according to an eighth embodiment ofthe present invention;

[0060] FIGS. 26A-26D and 27A-27C are sectional views illustrating amicromirror unit fabrication method according to a ninth embodiment ofthe present invention;

[0061] FIGS. 28A-28D and 29A-29D are sectional views illustrating amodified version of a micromirror unit fabrication method according tothe ninth embodiment of the present invention, in which a torsion barand a comb-teeth electrode are made in the same etching process;

[0062] FIGS. 30A-30E, 31A-31E, 32A-32E and 33A-33F are sectional viewsillustrating a modified version of a micromirror unit fabrication methodaccording to the ninth embodiment of the present invention, in which atorsion bar and a comb-teeth electrode are made in different etchingprocess;

[0063] FIGS. 34A-34C are sectional views illustrating a micromirror unitaccording to a tenth embodiment of the present invention;

[0064] FIGS. 35A-35E and 36A-36E are sectional views illustrating amicromirror unit fabrication method according to an eleventh embodimentof the present invention;

[0065] FIGS. 37A-37D and 38A-38D are sectional views illustrating amicromirror unit fabrication method according to an twelfth embodimentof the present invention;

[0066]FIG. 39 is a perspective view showing a conventional opticalswitching apparatus;

[0067]FIG. 40 is a perspective view showing a different type ofconventional optical switching apparatus;

[0068]FIG. 41 is an exploded view showing a conventional flat electrodetype micromirror unit;

[0069]FIG. 42 is an exploded view showing the micromirror unit of FIG.41 in which the inner frame is rotated through an angle θ relative tothe outer frame;

[0070]FIG. 43 is a perspective view illustrating a conventionalmicromirror unit provided with comb-teeth electrodes;

[0071]FIG. 44A is a perspective view showing some of the conductiveteeth of the comb-teeth electrodes of the micromirror unit shown in FIG.43 in which the upper three teeth extend in parallel to the lower twoteeth; and

[0072]FIG. 44B is a perspective view showing the skewed orientation ofthe upper teeth relative to the lower teeth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0073] The preferred embodiments of the present invention will bedescribed below with reference to the accompanying drawings.

[0074] FIGS. 1A-1B and 2A-2C show a micromirror unit 100 according to afirst embodiment of the present invention. FIG. 1A is an upper-side planview of the unit 100, while FIG. 1B is a lower-side plan view of theunit 100. FIGS. 2A-2C are sectional views taken along lines A-A, B-B andC-C, respectively, in FIGS. 1.

[0075] As shown in FIGS. 1A and 1B, the micromirror unit 100 includes amirror forming base 110, an inner frame 120 (surrounding the base 110),an outer frame 130 (surrounding the inner frame 120), a pair of firsttorsion bars 140 (connecting the mirror forming base 110 to the innerframe 120) and another pair of second torsion bars 150 (connecting theinner frame 120 to the outer frame 130). The first torsion bars 140define a first rotation axis X1 about which the mirror forming base 110is rotated relative to the inner frame 120. The second torsion bars 150define a second rotation axis X2 about which the inner frame 120(together with the mirror forming base 110) is rotated relative to theouter frame 130. In the illustrated example, the first axis X1 and thesecond axis X2 are generally perpendicular to each other. Themicromirror unit 100 as a whole is made of a conductive material exceptfor a mirror surface 111 and an insulating layer 160 (to be describedlater). A conductive material may be silicon (or polysilicon) doped withn-type impurity (e.g. phosphorus or arsenic) or p-type impurity (e.g.boron).

[0076] As shown in FIG. 1A, the mirror forming base 110 has an uppersurface upon which a mirror member 111 is formed. Also, the base 110includes two opposing side surfaces from which first comb-teethelectrodes 110 a, 110 b extend laterally.

[0077] As shown in FIG. 1B and FIGS. 2, the inner frame 120 has amulti-layer structure composed of an inner frame body 121, a pair ofelectrode bases 122 and an insulating layer 160 disposed between thebody 121 and the bases 122. The insulating layer 160 is made of siliconoxide. Each of the electrode bases 122 is formed integral with a secondcomb-teeth electrode 122 a or 122 b extending inward. The inner framebody 121 is formed integral with third comb-teeth electrodes 121 a and121 b extending outward. As shown in FIG. 2A, the second comb-teethelectrodes 122 a, 122 b are disposed below the first comb-teethelectrodes 110 a, 110 b. When the mirror forming base 110 is driven, thefirst comb-teeth electrodes 110 a, 110 b may be moved downward. To avoidthe interference between the first and the second comb-teeth electrodes,the first comb-teeth electrodes 110 a, 110 b are laterally offset fromthe second comb-teeth electrodes 122 a, 122 b, as shown in FIG. 2C.

[0078] As shown in FIG. 2B, the first torsion bars 140 are made smallerin thickness than the mirror forming base 110. The inner end of each bar140 is connected to the mirror forming base 110, while the outer end ofthe bar 140 is connected to the inner frame body 121.

[0079] As shown in FIG. 2A, the outer frame 130 is composed of a firstouter frame member 131, a second outer frame member 132 and aninsulating layer 160 disposed between the first and the second outerframe members. The second outer frame member 132, as shown in FIG. 1B,is provided with a first island 133, a second island 134, a third island135 and a fourth island 136 all of which are separated from each otherby an gap. As shown in FIGS. 2B and 2C, each of the first island 133 andthe third island 135 is formed integral with a fourth comb-teethelectrode 132 a or 132 b extending inward. The fourth comb-teethelectrodes 132 a, 132 b are disposed below the third comb-teethelectrodes 121 a, 121 b of the inner frame body 121. Laterally, thefourth and the third comb-teeth electrodes are offset from each other sothat they do not interfere with each other in operation.

[0080] As shown in FIG. 2A, each of the second torsion bars 150 isconnected, at its inner end, to a portion of the inner frame body 121that is held in contact with the insulating layer 160, and alsoconnected, at its outer end, to a portion of the first outer framemember 131 that is held in contact with the insulating layer 160.

[0081] In the illustrated example, when voltage is applied to the firstouter frame member 131, the first comb-teeth electrodes 110 a, 110 b andthe third comb-teeth electrodes 121 a, 121 b are held at the sameelectrical potential. This is because the first outer frame member 131,the second torsion bars 150, the inner frame body 121, the first torsionbars 140 and the mirror forming base 110 are integrally made of the samesilicon-base material. With the above-mentioned electrodes held at thesame potential, the second comb-teeth electrode 122 a or 122 b ischarged accordingly so that an electrostatic force is generated betweenthe first comb-teeth electrode 110 a and the second comb-teeth electrode122 a, or , between the first comb-teeth electrode 110 b and the secondcomb-teeth electrode 122 b, thereby rotating the mirror forming base 110about the first axis X1. As readily understood, to rotate the mirrorforming base 110 together with the inner frame 120 about the second axisX2, an electrostatic force is generated between the third comb-teethelectrode 121 a and the fourth comb-teeth electrode 132 a, or , betweenthe third comb-teeth electrode 121 b and the fourth comb-teeth electrode132 b. To this end, the fourth comb-teeth electrode 132 a or 132 b ischarged to a predetermined potential.

[0082] Reference is now made to FIGS. 3A-3D, 4A-4E, 5A-5D, 6A-6D and7A-7D illustrating a micromirror unit fabrication method according to asecond embodiment of the present invention. This method employs amicro-machining technique and can produce the above-describedmicromirror unit 100 in the following manner. It should be noted thateach of the figures presents a sectional view which appears to be flat,though this is done only for simplicity of illustration. In reality, thesection is not taken linearly but taken zigzaggedly. More specifically,a mirror forming portion M represents a sectional view of the mirrorforming base 110. Likewise, a torsion bar portion T represents asectional view of the first torsion bars 140 and 150. A comb-teethelectrode portion El represents a sectional view of the first and thesecond comb-teeth electrodes 110 a, 110 b, 122 a and 122 b. A comb-teethelectrode portion E2 represents a sectional view of the third and thefourth comb-teeth electrodes 121 a, 121 b, 132 a and 132 b. The top anddown of each sectional view shown in FIGS. 3-7 are opposite to those ofthe sectional views shown in FIGS. 2.

[0083] In fabricating a micromirror unit 100, to begin with, a first SOI(Silicon on Insulator) wafer 1 is prepared. The wafer 1 has amulti-layer structure composed of a relatively thin first silicon layer11, a relatively thick second silicon layer 12 and an insulating layer160 held between the first and the second silicon layers 11, 12. Thefirst silicon layer 11 is made of a conductive silicon material (orpolysilicon material) doped with n-type impurity (e.g. phosphorus orarsenic). The second silicon layer 12 is also made of a conductivesilicon material doped with n-type impurity (e.g. phosphorus orarsenic). For providing conductivity, the silicon material may be dopedwith p-type impurity (e.g. boron) The insulating layer 160 is made up ofsilicon oxide produced in a surface of the first or second silicon layer11 or 12 by thermal oxidation or CVD (chemical vapor deposition). Afterthe insulating layer 160 is produced, the first silicon layer 11 and thesecond silicon layer 12 are fixed to each other. Thus, the first SOIwafer 1 is obtained. In the illustrated example, the first silicon layer11 has a thickness of 5 μm, the second silicon layer 12 has a thicknessof 100 μm, and the insulating layer 160 has a thickness of 1 μm.

[0084] Then, a film of silicon oxide is grown on the second siliconlayer 12 by thermal oxidation. This oxide film is subjected topatterning, as shown in FIG. 3B, so that an oxide film pattern 51 isformed. The patterning of the oxide film may be performed with the useof an etchant such as buffered hydrofluoric acid (produced by DaikinIndustries, Ltd.) composed of hydrofluoric acid and ammonium fluoride.This etchant may be used for the subsequent oxide film patterning (to bedescribed later). The oxide film pattern 51 covers portions of thesecond silicon layer 12 that will be processed into a comb-teethelectrode portion E2 and a frame. More specifically, referring to FIG.1B, the pattern 51 has been configured in a manner corresponding to theelectrode bases 122, the second comb-teeth electrodes 122 a-122 b, thefirst to the fourth islands 133-136 of the second outer frame member 132and the fourth comb-teeth electrodes 132 a-132 b. Further, a photoresistfilm is formed on the first silicon layer 11 by spin coating. This filmis processed into a resist pattern 52 by exposure of light anddevelopment. An example of photoresist material may be AZP4210 (producedby Clariant Japan) or AZ1500 (produced by Clariant Japan). The samematerial may be used for the subsequent similar procedures. The resistpattern 52 covers portions of the first silicon layer 11 that correspondto the mirror forming portion M, the torsion bar portion T, thecomb-teeth electrode portion El and the frame forming portion.

[0085] Then, as shown in FIG. 3C, the first silicon layer 11 issubjected to DRIE (Deep Reactive Ion Etching) using the resist pattern52 as a mask until the etching reaches the insulating layer 160. In theDRIE, the “Bosch process” is adopted, whereby etching and side wallprotection are alternatively performed. Specifically, a first etchingstep may be performed for 8 seconds by using SF6 gas, and then a firstside wall protection step may be performed for 6.5 seconds by using C4F8gas. Thereafter, these two steps may be repeated several times. In theprocess, a bias of 23 watts is applied to the wafer for attaining properetching.

[0086] The above-described Bosch process may be adopted in thesubsequent DRIE for a silicon layer and a polysilicon layer. Instead ofDRIE, wet etching with the use of e.g. KOH solution may be performed. Itis also possible to employ inductively coupled plasma etching, wherebyetching is performed in high density plasma.

[0087] As the result of the etching process to the first silicon layer11, a pre-torsion bar T′ (which will become a torsion bar T later) isformed. The pre-torsion bar t′ is held in contact with the insulatinglayer 160 and has a thickness of 5 μm. After the pre-torsion bar T′ isformed, the resist pattern 52 is removed by using an exfoliationsolution such as AZ Remover 200 (produced by Clariant Japan) The sameexfoliation solution may be used for the subsequent removal of resistpatterns.

[0088] Then, as shown in FIG. 3D, a third silicon layer 13 is attachedto the wafer 1 on the lower side (in the figure) where the pre-torsionbar T′ has been formed. Thus, a second SOI wafer 2, which incorporatesthe pre-torsion bar T′, is obtained. The attachment of the layer 13 isperformed under vacuum. At this time, the wafer 1 and the third siliconlayer 13 may preferably be heated up to 1100° C. The third silicon layer13 is made of a conductive silicon material doped with impurities andhas a thickness of 100 μm. In place of the third silicon layer 13, usemay be made of a silicon wafer having a thickness of 300 μm. Thisinitially thick wafer is attached to the wafer 1 and then subjected togrinding to be made 100 μm in thickness.

[0089] Then, as shown in FIG. 4A, an oxide film pattern 53 is formed onthe third silicon layer 13. The pattern 53 is made by patterning anoxide film which is grown on the layer 13 by thermal oxidation. Theoxide film pattern 53 covers portions of the third silicon layer 13 thatcorrespond to the mirror forming portion M, the comb-teeth electrodeportion El and the frames. Specifically, the pattern 53 is configured inaccordance with the mirror forming base 110 (see FIG. 1A), the firstcomb-teeth electrodes 110 a-110 b, the inner frame body 121, the thirdcomb-teeth electrodes 121 a-121 b and the first outer frame member 131.

[0090] Then, as shown in FIG. 4B, a photoresist pattern 54 is formed onthe second silicon layer 12. The pattern 54 may be made by subjecting aphotoresist film to exposure of light and development. The pattern 54covers the second silicon layer 12 except for a region thereofcorresponding to the mirror forming portion M. For simplicity ofillustration, referring to FIG. 4B, the resist pattern 54 is depicted asbeing spaced from the second silicon layer 12. In reality, however, thepattern 54, covering the oxide film pattern 51, is held in contact withthe second silicon layer 12. The same simplicity scheme holds for thesubsequent depiction of a resist pattern formed on the silicon layers.

[0091] Then, as shown in FIG. 4C, the second silicon layer 12 issubjected to DRIE, with the resist pattern 54 used as a mask. The DRIEcontinues until the etching reaches the insulating layer 160. As aresult, the uncovered portion of the layer 12 is etched away, therebyexposing a part of the insulating layer 160. The insulating layer 160remains substantially intact through the DRIE using SF6 gas and C4F8gas.

[0092] Then, as shown in FIG. 4D, the exposed part of the insulatinglayer 160 is removed by etching, whereby the first silicon layer 11 ispartially exposed. The etchant suitable for the layer 160 (made ofsilicon oxide) may be buffered hydrofluoric acid composed ofhydrofluoric acid and ammonium fluoride.

[0093] Then, as shown in FIG. 4E, the exposed part of the first siliconlayer 11 and the underlying part of the third silicon layer 13 areremoved by DRIE. In this manner, a part of the mirror forming portion Mis reduced in thickness by a predetermined amount. Due to the partialreduction in thickness, the mirror forming portion M is reduced inweight. This is advantageous to improving the speed of the operation ofthe mirror forming base 110 in a product micromirror unit 100.

[0094] Then, the resist pattern 54 is removed. Next, as shown in FIG.5A, a photoresist 55′ is formed by spraying a photoresist solution fromabove. The photoresist solution may be AZP4210 (produced by ClariantJapan) which is diluted with AZ5200 thinner (produced by ClariantJapan). The concentration of the resulting solution may be one fifth ofthe original concentration of the AZP4210 (five-times dilution).

[0095] Then, the photoresist 55′ is exposed to light and developed, tobe processed into a resist pattern 55, as shown in FIG. 5B.Specifically, the photoresist 55′ on the second silicon layer 12 isremoved at this stage. The resist pattern 55 covers the bottom side ofthe mirror forming portion M in the third silicon layer 13.

[0096] Then, as shown in FIG. 5C, the second silicon layer 12 issubjected to DRIE with the oxide film pattern 51 used as a mask untilthe etching reaches the insulating layer 160. As a result, thecomb-teeth electrode E2 is formed. At this stage, the mirror formingportion M is not affected by the etchant due to the masking of theresist pattern 55. Thereafter, as shown in FIG. 5D, the resist pattern55 is removed from the bottom side of the mirror forming portion M.

[0097] Then, as shown in FIG. 6A, a photoresist 56′ is formed on thethird silicon layer 13 by spin coating. The photoresist 56′ is thenexposed to light and developed, to be processed into a resist pattern56, as shown in FIG. 6B. The resist pattern 56 covers regions of thethird and the first silicon layers 13, 11 that correspond to the mirrorforming portion M, the comb-teeth electrode El and the frame.

[0098] Then, as shown in FIG. 6C, the third and the first silicon layers13, 11 are subjected to DRIE with the resist pattern 56 used as a maskuntil the pre-torsion bar T′ is exposed. Thereafter, as shown in FIG.6D, the resist pattern 56 is removed.

[0099] Then, as shown in FIG. 7A, a photoresist 57′ is formed byspraying the material from below (in FIG. 7A). The photoresist 57′ isthen subjected to exposure of light and development, to be processedinto a resist pattern 57, as shown in FIG. 7B. The resist pattern 57covers the pre-torsion bar T′.

[0100] Then, as shown in FIG. 7C, the third and the first silicon layers13, 11 are subjected to DRIE with the oxide film pattern 53 used as amask until the etching reaches the insulating layer 160. As a result,the comb-teeth electrode El is obtained. At this stage, the pre-torsionbar T′ is not affected by the etching due to the masking of the resistpattern 57.

[0101] Then, after the resist pattern 57 is removed, the exposed part ofthe insulating layer 160 is etched away, as shown in FIG. 7D, byimmersion in the etchant. At this time, the exposed oxide film patterns51, 53 are also removed. As a result, upper and lower comb-teethelectrodes E1, E2 having a thickness of 100 μm are obtained. Also, athin torsion bar T is obtained. The torsion bar T is located exactly atthe vertical center of the second SOI wafer 2 and has a precisethickness of 5 μm. In addition, a mirror forming portion M, which ispartially reduced in thickness, is obtained. Accordingly, a micromirrorunit 100 drivable with low power is obtained.

[0102] Reference is now made to FIGS. 8A-8E, 9A-9E, 10A-11D, 11A-11D and12A-12D illustrating a micromirror unit fabrication method according toa third embodiment of the present invention. In the illustrated example,the method is used for making the above-described micromirror unit 100.

[0103] According to the fabrication method of the third embodiment, tobegin with, a first SOI wafer 1 (FIG. 8A) is prepared through the samesteps as described above with reference to FIGS. 3A-3C. The wafer 1shown in FIG. 8A includes a first silicon layer 11, a second siliconlayer 12, an oxide film pattern 51, a resist pattern 52 and apre-torsion bar T′. The oxide film pattern 51 is formed on the secondsilicon layer 12. The pre-torsion bar T′ is formed in the first siliconlayer 11 by DRIE with the resist pattern 52 used as a mask. Thepre-torsion bar T′ has a thickness of 5 μm.

[0104] Then, the resist pattern 52 is removed. Thereafter, an oxide filmis formed on the first silicon layer 11 by thermal oxidation. This filmis processed into an oxide film pattern 58, as shown in FIG. 8B. Thepattern 58 is used for the masking of a pre-torsion bar T′ at a latteretching process.

[0105] According to the illustrated method, a silicon wafer 13′ (seeFIG. 8C) is also prepared together with the first wafer 1. (After beingattached to the first wafer 1, the silicon wafer 13′ may be referred toas a third silicon layer 13.) A photoresist film is formed on the wafer13′ and subjected to light exposure and development. Thus, as shown inFIG. 8C, a resist pattern 59 is obtained. The wafer 13′ is made of aconductive silicon material doped with an impurity and has a thicknessof 100 μm. The resist pattern 59 covers regions of the wafer 13′ thatcorrespond to a mirror forming portion M, a comb-teeth electrode E1 anda frame.

[0106] Then, as shown in FIG. 8D, the wafer 13′ is subjected to DRIEwith the resist pattern 59 used as a mask until the etching reaches apredetermined depth. As a result, a groove 13 a is formed. Then, theresist pattern 59 is removed.

[0107] Then, as shown in FIG. 8E, the wafer 13′ is attached to the firstsilicon layer 11 of the first wafer 1 under vacuum. For properattachment, the wafers 1 and 13′ are heated up to 1100° C. The firstwafer 1 is positioned to the counterpart wafer 13′ so that thepre-torsion bar T′ faces the groove 13 a formed in the wafer 13′. Inthis manner, a second SOI wafer 2 including a third silicon layer 13 anda concealed pre-torsion bar T′ is obtained.

[0108] Then, an oxide film is formed on the third silicon layer 13 bythermal oxidation and then processed into a oxide film pattern 53, asshown in FIG. 9A. The pattern 53 is configured in accordance the plangeometry involving the mirror forming base 110, the first comb-teethelectrodes 110 a-110 b, the inner frame body 121, the third comb-teethelectrodes 121 a-121 b and the first outer frame member 131.

[0109] Then, as shown in FIG. 9B, a resist pattern 54 is formed bygrowing a photoresist on the second silicon layer 12 and then subjectingthe photoresist to light exposure and development. Except for the regioncorresponding to the mirror forming portion M, the resist pattern 54covers all the other regions in the upper surface of the second siliconlayer 12. Then, as shown in FIG. 9C, the second silicon layer 12 issubjected to DRIE with the resist pattern 54 used as a mask until theetching reaches the insulating layer 160. Then, as shown in FIG. 9D, theexposed portion of the insulating layer 160 is etched away. Then, asshown in FIG. 9E, the first and the third silicon layers 11, 13 aresubjected to DRIE, thereby reducing the thickness of a part of themirror forming portion M. As seen from the comparison of the relevantfigures, the procedure shown in FIGS. 9A-9E is similar to the procedureshown in FIGS. 4A-4E (second embodiment of the present invention).

[0110] Then, after the resist pattern 54 is removed, a photoresist 55′is formed by spraying the material from above, as shown in FIG. 10A. Thephotoresist 55′ is then exposed to light and developed, to be processedinto a resist pattern 55, as shown in FIG. 10B. The resist pattern 55covers the bottom side of the mirror forming portion M in the thirdsilicon layer 13. Then, as shown in FIG. 10C, the first silicon layer 11is subjected to DRIE with the oxide film pattern 51 used as a mask untilthe etching reaches the insulating layer 160. As a result, thecomb-teeth electrode E2 is produced. Then, as shown in FIG. 10D, theresist pattern 55 is removed. The procedure shown in FIGS. 10A-10D issimilar to the procedure shown in FIGS. 5A-5D (second embodiment of thepresent invention).

[0111] Then, as shown in FIG. 11A, a photoresist 56′ is formed on thethird silicon layer 13 and then exposed to light and developed, to beprocessed into a resist pattern 56, as shown in FIG. 11B. The resistpattern 56 covers regions of the third and first silicon layers 13, 11that correspond to the mirror forming portion M, the comb-teethelectrode El and the frame. Then, as shown in FIG. 1C, the third and thefirst silicon layers 13, 11 are subjected to DRIE with the resistpattern 56 used as a mask until the pre-torsion bar T′ is revealed. Dueto the covering of the oxide film pattern 58, the pre-torsion bar T′ isnot etched away. Then, as shown in FIG. 11D, the resist pattern 56 isremoved. The procedure shown in FIGS. 11A-11D is similar to theprocedure shown in FIGS. 6A-6D.

[0112] Then, as shown in FIG. 12A, a photoresist 57′ is formed byspraying the material from below. The photoresist 57′ is then exposed tolight and developed, to be processed into a resist pattern 57, as shownin FIG. 12B. The pattern 57 serves as an additional mask for thepre-torsion bar T′. Then, as shown in FIG. 12C, the third and the firstsilicon layers 13, 11 are subjected to DRIE with the oxide film pattern53 used as a mask until the etching reaches the insulating layer 160.Thus, the comb-teeth electrode E1 is obtained. At this time, thepre-torsion bar T′ is not affected by the etchant due to the doublemasking by the pattern 58 and the pattern 57. Then, as shown in FIG.12D, the resist pattern 57 is removed, and the exposed insulating layer160 and pattern 58 are etched away. At this time, the exposed oxide filmpatterns 51, 53 are also removed. As a result, upper and lowercomb-teeth electrodes E1, E2 having a thickness of 100 μm are obtained.The resulting torsion bar T is advantageously thin (having a thicknessof Sum with high precision), and is disposed accurately at the verticalcenter of the second SOI wafer 2. The procedure shown in FIGS. 12A-12Dis similar to the procedure shown in FIGS. 7A-7D (second embodiment ofthe present invention).

[0113] Reference is now made to FIGS. 13A-13E, 14A-14E, 15A-15D, 16A-16Dand 17A-17D illustrating a micromirror unit fabrication method accordingto a fourth embodiment of the present invention. In the illustratedexample, the method is used for making the above-described micromirrorunit 100.

[0114] First, as shown in FIG. 13A, a first SOI wafer 1 is prepared. Theillustrated water is the same as the one shown in FIG. 3A (secondembodiment of the present invention). Then, as shown in FIG. 13B, anoxide film pattern 51 is formed on the second silicon layer 12 bygrowing a silicon oxide film on the layer 12 (by thermal oxidation) andthen patterning this oxide film. Another oxide film 58′ is grown on thefirst silicon layer 11. However, the oxide film 58′ is not subjected topatterning.

[0115] Then, a photoresist is formed on the oxide film 58′, and thensubjected to patterning to form a resist pattern 60, as shown in FIG.13C. Then, the oxide film 58′ is etched with the resist pattern 60 usedas a mask. Thereafter, the resist pattern 60 is removed. Thus, as shownin FIG. 13D, on oxide film pattern 58 is obtained. The pattern 58 willserve as a mask for a pre-torsion bar T′ at a subsequent etching step.

[0116] Then, a silicon water 13′ is prepared in the same manner asdescribed with reference to FIGS. 8C and 8D (third embodiment of thepresent invention). After the resist pattern 59 is removed, the siliconwafer 13′ is attached to the first SOI wafer 1 so that the pattern 58faces the groove 13 a of the wafer 13′, as shown in FIG. 13E.

[0117] Then, an oxide film is grown on the third silicon layer 13 bythermal oxidation. Then, as in the first embodiment of the presentinvention, the oxide film is processed into an oxide film pattern 53! asshown in FIG. 14A.

[0118] Then, a photoresist is formed on the second silicon layer 12.This photoresist is exposed to light and developed, to be processed intoa resist pattern 54, as shown in FIG. 14B. The pattern 54 serves as amask for the second silicon layer 12, but does not cover a region of thelayer 12 that corresponds to the mirror forming portion M. Then, asshown in FIG. 14C, the second silicon layer 12 is subjected to DRIE withthe resist pattern 54 used as a mask until the etching reaches theinsulating layer 160. Then, as shown in FIG. 14D, the exposed portion ofthe insulating layer 160 is etched away. Then, as shown in FIG. 14E, theexposed first silicon layer 11 and the underlying third silicon layer 13are subjected to DRIE, thereby partially reducing the thickness of themirror forming portion M. The procedure shown in FIGS. 14A-14E issimilar to the procedure shown in FIGS. 4A-4E (second embodiment of thepresent invention).

[0119] Then, the resist pattern 54 is removed. Thereafter, as shown inFIG. 15A, a photoresist 55′ is formed by spraying the material fromabove. The photoresist 55′ is then exposed to light and developed, to beprocessed into a resist pattern 55, as shown in FIG. 15B. The resistpattern 55 serves as a mask for the bottom side of the mirror formingportion M in the third silicon layer 13. Then, as shown in FIG. 15C, thefirst silicon layer 11 is subjected to DRIE with the pattern 51 used asa mask until the etching reaches the insulating layer 160. Thus, thecomb-teeth electrode E2 is obtained. Then, as shown in FIG. 15D, theresist pattern 55 is removed. The procedure shown in FIGS. 15A-15D issimilar to the procedure shown in FIGS. 5A-5D (second embodiment of thepresent invention.

[0120] Then, as shown in FIG. 16A, a photoresist 56′ is formed on thethird silicon layer 13. The photoresist 56′ is exposed to light anddeveloped, to be processed into a resist pattern 56, as shown in FIG.16B. The resist pattern 56 serves as a mask for regions of the third andthe first silicon layers 13, 11 that correspond to the mirror formingportion M, the comb-teeth electrode El and the frame. Then, as shown inFIG. 16C, the third and the first silicon layers 13, 11 are subjected toDRIE with the pattern 56 used as a mask. The etching continues until theoxide film 58 is revealed and further the insulating layer 160 isexposed. As a result, a pre-torsion bar T′ is obtained. At this stage,the pre-torsion bar T′ is held in contact with the insulating layer 160.Due to the protection by the oxide film 58, the pre-torsion bar T′remains intact during the DRIE. Then, as shown in FIG. 16D, the resistpattern 56 is removed. The procedure shown in FIGS. 16A-16D is similarto the procedure shown in FIGS. 6A-6D (second embodiment of the presentinvention).

[0121] Then, as shown in FIG. 17A, a photoresist 57′ is formed byspraying the material from below. The photoresist 57′ is exposed tolight and developed, to be processed into a resist pattern 57, as shownin FIG. 17B. The resist pattern 57 serves as an additional mask for thepre-torsion bar T′. Then, as shown in FIG. 17C, the third silicon layer13 and the underlying first silicon layer 11 are subjected to DRIE withthe oxide pattern 53 used as a mask until the etching reaches theinsulating layer 160. Thus, the comb-teeth electrode El is obtained. Atthis stage, the pre-torsion bar T′ is masked doubly by the patterns 58and 57, so that it can remain intact during the etching. Then, theresist pattern 57 is removed. Thereafter, as shown in FIG. 17D, theexposed insulating layer 160 and the oxide film pattern 58 are etchedaway. At this time, the oxide film patterns 51, 53 are also etched away.As a result, upper and lower comb-teeth electrodes E1, E2 having athickness of 100 μm are obtained. Also, a thin torsion bar T isobtained. The torsion bar T is located exactly at the vertical center ofthe second SOI wafer 2 and has a precise thickness of 5 μm. In addition,a mirror forming portion M, which is partially reduced in thickness, isobtained. Accordingly, a micromirror unit 100 drivable with low power isobtained. The procedure shown in FIGS. 17A-17D is similar to theprocedure shown in FIGS. 7A-7D.

[0122] Reference is now made to FIGS. 18A-18D and 19A-19D illustrating amicromirror unit fabrication method according to a fifth embodiment ofthe present invention. This method also employs a micro-machiningtechnique and can produce the above-described micromirror unit 100.

[0123] First, as shown in FIG. 18A, a mask pattern 61 is formed on anSOI wafer 3 by using an oxide film, a photoresist etc. The wafer 3 has amulti-layer structure composed of a first silicon layer 14, a secondsilicon layer 15 and an insulating layer 160 held between the first andthe second layers 14, 15. Each of the first and the second siliconlayers 14, 15 is made of a conductive silicon material doped with n-typeimpurities such as phosphorus or arsenic. The insulating layer 160 ismade of silicon oxide grown on the layer 14 or 15 by thermal oxidation.In the illustrated example, each of the first and the second siliconlayers 14, 15 has a thickness of 100 μm, while the insulating layer 160has a thickness of 1 μm.

[0124] Then, as shown in FIG. 18B, the first silicon layer 14 issubjected to DRIE with the presence of the mask pattern 61 until theetching reaches the insulating layer 160. Thus, a groove 14 a isobtained. Then, the mask pattern 61 is removed. Then, as shown in FIG.18C, the SOI wafer 3 as a whole is coated by polysilicon layers 14′ and15′ each having a thickness of 5 μm. These layers may be produced byreduced-pressure chemical vapor deposition. In the illustrated example,the polysilicon to be used is doped with impurities so that it isrendered electrically conductive. Then, an oxide film is grown on thepolysilicon layer 15′ by thermal oxidation. The thus obtained oxide filmis then processed into an oxide film pattern 62, as shown in FIG. 18D.

[0125] Then, as shown in FIG. 19A, the second silicon layer 15 and thepolysilicon layer 15′ are subjected to DRIE with the oxide film pattern62 used as a mask until the etching reaches the insulating layer 160.Then, as shown in FIG. 19B, the exposed insulating layer 160 is etchedaway. Then, as shown in FIG. 19C, portions of the polysilicon layer 14′that are exposed on the side of the second silicon layer 15 are removedby DRIE. Thus, a pre-torsion bar T′ held in contact with the insulatinglayer 160 is obtained. Then, as shown in FIG. 19D, etching is performedso that the portion of the insulating layer 160 that is held in contactwith the pre-torsion bar T′ is removed. More specifically, the SOI wafer3 is immersed in etchant. As a result, the above-mentioned particularportion of the insulating layer 160 is dissolved. Accordingly, it ispossible to remove the portion of the second silicon layer 15 that isheld in contact with the dissolved insulating layer portion. At thistime, the exposed oxide film pattern 62 is also removed. Thus, thetorsion bar T is obtained.

[0126] In the above embodiment again, first a pre-torsion bar T′ held incontact with an insulating layer 160 is produced, and then the attachedinsulating layer 160 is removed from the pre-torsion bar T′. Thethickness of the pre-torsion bar T′ is determined by the thickness ofthe polysilicon layer 14′ formed at the step shown in FIG. 18C. Thethickness of the layer 14′ can be precisely determined by a film formingtechnique. Accordingly, it is possible to set the thickness of thetorsion bar T at a desired small value with high precision.

[0127] Reference is now made to FIGS. 20A-20D and 21A-21D illustrating amicromirror unit fabrication method according to a sixth embodiment ofthe present invention. This method also employs a micro-machiningtechnique and can produce the above-described micromirror unit 100.

[0128] In the illustrated embodiment, an SOI wafer 3 is subjected to thesame process as shown in FIGS. 18A-18C (fifth embodiment of the presentinvention), so that a processed wafer 3 shown in FIG. 20A is obtained.The illustrated wafer 3 includes a first silicon layer 14, a secondsilicon layer 15 and an insulating layer 160 disposed between the firstand the second silicon layers 14 and 15. The first silicon layer 14 isformed with a groove 14 a. The wafer 3 as a whole is coated withpolysilicon layers 14′ and 15′.

[0129] Then, as shown in FIG. 20B, a photoresist 63′ is formed on thepolysilicon layer 14′. The photoresist 63′ is then processed into aresist pattern 63, as shown in FIG. 20C. The resist pattern 63 serves asa mask for producing a torsion bar T in the polysilicon layer 14′. Then,the exposed portions of the polysilicon layer 14′ are removed, as shownin FIG. 20D, by DRIE with the resist pattern 63 used as a mask. Thus, apre-torsion bar T′ held in contact with the insulating layer 160 isobtained.

[0130] Then, as shown in FIG. 21A, the resist pattern 63 is removed.Then, a resist 64′ is formed on the polysilicon layer 15′, as shown inFIG. 21B. This resist is then processed into a resist pattern 64. Then,the second silicon layer 15 and the polysilicon layer 15′ are subjectedto DRIE with the resist pattern 64 used as a mask until the etchingreaches the insulating layer 160, as shown in FIG. 21C. Then, as shownin FIG. 21D, a portion of the insulating layer 160 that is held incontact with the pre-torsion bar T′ is etched away. Thus, a torsion barT is obtained. Thereafter, the resist pattern 64 is removed, asrequired.

[0131] Reference is now made to Figs. FIGS. 22A-22E and 23A-23Dillustrating a micromirror unit fabrication method according to aseventh embodiment of the present invention. This method again employs amicro-machining technique and can produce the above-describedmicromirror unit 100.

[0132] In the illustrated embodiment, as shown in FIG. 22A, first agroove 16 a is formed in a silicon wafer 4. Then, an oxide film 65′ isformed on the wafer 4 so as to cover the groove 16 a. In addition, apolysilicon layer 16′ is formed on the film 65′ in a manner such that apart of the layer 16′ fills the groove 16 a. Then, as shown in FIG. 22B,the oxide film 65′ and the polysilicon layer 16′ are removed by grindingexcept for the projecting portion filling the groove 16 a. Then, asshown in FIG. 22C , oxide films 66′ and 67′ are grown on the upper orlower surface of the wafer 4 by thermal oxidation. The oxide film 66′will serve as an intermediate insulating layer 160. The projectingportion of the polysilicon 16′ serves as a pre-torsion bar T′. Then, asshown in FIG. 22D, a second silicon layer 17 is attached to the wafer 4with the film 66′ intervening therebetween. Thus, an SOI wafer 5 havinga multi-layer structure is obtained. Then, the oxide film 67′ isprocessed into an oxide film pattern 67, as shown in FIG. 22E.

[0133] Then, as shown in FIG. 23A, the first silicon layer 16 issubjected to DRIE with the pattern 67 used as a mask until the oxidefilm 65′ formed in the groove 16 a is exposed. Then, an oxide film isformed on the second silicon layer 17 and then processed into an oxidefilm pattern 68, as shown in FIG. 23B. Then, as shown in FIG. 23C, thesecond silicon layer 17 is subjected to DRIE with the pattern 68 used asa mask until the etching reaches the insulating layer 160. Then, asshown in FIG. 23D, the exposed insulating layer 160 and the oxide film65′ are etched away by immersing the SOI wafer in etchant. At this time,the oxide film patterns 67 and 68 are also removed. Thus, a torsion barT is obtained.

[0134] Reference is now made to FIGS. 24A-24D and 25A-25C illustrating amicromirror unit fabrication method according to an eighth embodiment ofthe present invention. This method again employs a micro-machiningtechnique and can produce the above-described micromirror unit 100.

[0135] In the illustrated embodiment, first a silicon wafer 6, whichwill serve as a first silicon layer 18 at a latter stage, is prepared.As shown in FIG. 24A, the wafer 6 is formed with two grooves by etchingwith the use of a suitable mask pattern (not shown). This mask patterncovers a portion of the wafer 6 that will become a torsion bar T. Thedepth of each groove is rendered equal to the thickness of the torsionbar T to be obtained. Then, as shown in FIG. 24B, a second silicon layer19 provided with an oxide film 69′ is attached to the wafer 6. Thus, anSOI wafer 7 is obtained. The oxide film 69′ serves as an intermediateinsulating layer 160 in the wafer 7. Then, as shown in FIG. 24C, anoxide film pattern 70 is formed on the second silicon layer 19. Then, asshown in FIG. 24D, the second silicon layer 19 is subjected to DRIE withthe pattern 70 used as a mask until the etching reaches the insulatinglayer 160.

[0136] Then, as shown in FIG. 25A, an oxide film pattern 71 is formed onthe first silicon layer 18. Then, as shown in FIG. 25B, the firstsilicon layer 18 is subjected to DRIE with the pattern 71 used as a maskuntil a pre-torsion bar T′ held in contact with the insulating layer 160is obtained. Then, as shown in FIG. 25C, the insulating layer 160contacting with the bar T′ is etched away. At this time, the oxide filmpatterns 70, 71 are also removed. Thus, a torsion bar T is obtained.

[0137] Reference is now made to FIGS. 26A-26E and 27A-27C illustrating amicromirror unit fabrication method according to a ninth embodiment ofthe present invention. This method again employs a micro-machiningtechnique and can produce the above-described micromirror unit 100.

[0138] In the illustrated embodiment, as shown in FIG. 26A, an oxidefilm pattern 72 is formed on an SOI wafer 8. The SOI wafer 8 has amulti-layer structure composed of a first silicon layer 20, a secondsilicon layer 21 and an intermediate insulating layer 160 disposedbetween the first and the second silicon layers 20, 21. Each of thefirst and the second silicon layers 20, 21 is made of a conductivesilicon material doped with n-type impurities such as phosphorus andarsenic. The insulating layer 160 is made of silicon oxide which isgrown on the surface of the first or second silicon layer 20 or 21 bythermal oxidation. In the illustrated embodiment, both the first and thesecond silicon layers 20, 21 have a thickness of 100 μm, while theinsulating layer 160 has a thickness of 1 μm.

[0139] Then, as shown in FIG. 26B, a resist pattern 73 is formed on theexposed area of the first silicon layer 20. The masked portion of thelayer 20 is processed into a torsion bar T. As shown in FIG. 26C, thefirst silicon layer 20 is subjected to DRIE with the patterns 72 and 73used as a mask (first etching step), thereby producing two identicalgrooves. The depth of each groove corresponds to the thickness of thetorsion bar T. In the illustrated embodiment, the depth of the groovesis 5 μm. Then, as shown in FIG. 26D, the resist pattern 73 is removed.As shown in FIG. 26E, the first silicon layer 20 is subjected to DRIEwith the oxide film pattern 72 used as a mask (second etching step)until the etching reaches the insulating layer 160. Thus, a pre-torsionbar T′ contacting with the insulating layer 160 is obtained.

[0140] Then, as shown in FIG. 27A, an oxide film pattern 74 is formed onthe second silicon layer 21. As shown in FIG. 27B, the second siliconlayer 21 is subjected to DRIE with the pattern 74 used as a mask untilthe etching reaches the insulating layer 160. Then, as shown in FIG.27C, the insulating layer 160 contacting with a pre-torsion bar T′ isetched away by immersion in etchant. At this time, the oxide filmpatterns 72, 74 are also removed. Thus, a torsion bar T is obtained.

[0141] Reference is now made to FIGS. 28A-28D and 29A-29D illustrating amodified version of the micromirror unit fabrication method according tothe ninth embodiment of the present invention. In this version,comb-teeth electrodes E1 and E2 are produced together with a torsion barT.

[0142] First, as shown in FIG. 28A, an SOI wafer 8 is prepared. As notedabove, the wafer 8 includes a conductive first silicon layer 20, aconductive second silicon layer 21 and an intermediate insulating layer160.

[0143] Then, oxide films are grown on the first and the second siliconlayers 20, 21 by thermal oxidation. The oxide films are then processedinto a pattern 72 or 74, as shown in FIG. 28B. The oxide film patterns72 and 74 serve as a mask in etching the first or second silicon layerto provide comb-teeth electrodes E1 and E2. As shown in FIG. 28C, aresist pattern 73 is formed on the first silicon layer 20. The pattern73 serves as a mask for the portion to be processed into a torsion barT. Then, as shown in FIG. 28D, the first silicon layer 20 is subjectedto DRIE with the patterns 72 and 73 used as a mask until grooves eachhaving a depth of 5 μm are formed.

[0144] Then, as shown in FIG. 29A, the resist pattern 73 is removed.Then, as shown in FIG. 29B, the first silicon layer 20 is subjected toDRIE with the pattern 72 used as a mask until the etching reaches theinsulating layer 160. Thus, a pre-torsion bar T′ contacting with thelayer 160 and also a comb-teeth electrode E2 contacting with the layer160 are obtained. Then, as shown in FIG. 29C, the second silicon layer21 is subjected to DRIE with the pattern 74 used as a mask until theetching reaches the insulating layer 160. Thus, a comb-teeth electrodeEl is obtained. Then, as shown in FIG. 29D, the insulating layer 160exposed at the steps of FIGS. 29B and 29C is etched away. At this time,the oxide film patterns 72, 74 are also removed. Thus, a torsion bar Tis obtained.

[0145] During the DRIE at the step of FIG. 29A, the difference betweenthe etching rate in the comb-teeth electrode region and the etching ratein the torsion bar region is negligible since the etching depth to beattained is relatively small (5 μm). However, during the DRIE at thestep of FIG. 29B, the etching depth to be attained is rather great(several ten micrometers). Accordingly, the etching rate tends to begreater in the torsion bar region than in the comb-teeth electroderegion due to the difference in extent of the openings. In light ofthis, preferably the etching depth to be attained at the step of FIG.29A may be set at a value greater than the prescribed thickness of thetorsion bar (5 μm in the illustrated embodiment). With this precaution,when the etching at the step of FIG. 29B reaches the insulating layer160, the resulting pre-torsion bar T′ will have the desired thickness (5μm).

[0146] Reference is now made to FIGS. 30A-30E, 31A-31E, 32A-32E and33A-33F illustrating a modified version of the micromirror unitfabrication method of the ninth embodiment of the present invention. Inaccordance with this modified version, a torsion bar T and comb-teethelectrodes E1, E2 are produced at different etching steps, as describedbelow.

[0147] First, as shown in FIG. 30A, an SOI wafer 8 is prepared. Theillustrated wafer 8 is the same as the wafer shown in FIG. 26A. Then,oxide films are grown on the first and the second silicon layers 20, 21by thermal oxidation. The obtained oxide films are then processed into apattern 72 or 74, as shown in FIG. 30B. Then, as shown in FIG. 30C, aresist pattern 75 is formed on the first silicon layer 20. The resistpattern 75 serves as a mask for a region to be processed into a torsionbar T and for regions to be processed into a mirror forming portion M,comb-teeth electrodes E1, E2 and frames. Then, as shown in FIG. 30D, thefirst silicon layer 20 is subjected to DRIE with the pattern 75 used asa mask until grooves each having a depth of 5 μm are formed in the layer20. Then, as shown in FIG. 30E, the resist pattern 75 is removed.

[0148] Then, as shown in FIG. 31A, a photoresist 76′ is formed on thefirst silicon layer 20. Then, the photoresist 76′ is exposed to lightand developed, to be processed into a resist pattern 76, as shown inFIG. 31B. The pattern 76 serves as a mask for regions to be processedinto a mirror forming portion M, comb-teeth electrodes E1, E2 or frames.Then, as shown in FIG. 31C is the first silicon layer 20 is subjected toDRIE with the pattern 76 as a mask until the etching reaches theinsulating layer 160. Thus, a pre-torsion bar T′ contacting with theinsulating layer 160 is obtained. Then, as shown in FIG. 31D, the resistpattern 76 is removed. Then, as shown in FIG. 31E, a photoresist 77′ isformed on the first silicon layer 20 and the pretorsion bar T′ byspraying the material from above.

[0149] Then, the photoresist 77′ is exposed to light and developed, tobe processed into a resist pattern 77, as shown in FIG. 32A. The pattern77 serves as a mask for the pre-torsion bar T′. Then, as shown in FIG.32B, the first silicon layer 20 is subjected to etching with the pattern72 used as a mask until the etching reaches the insulating layer 160.Thus, an comb-teeth electrode E2 contacting with the insulating layer160 is obtained. Then, as shown in FIG. 32C, the resist pattern 77 isremoved, while a photoresist 78′ is formed on the second silicon layer21. Then, as shown in FIG. 32D, the photoresist 78′ is processed into aresist pattern 78. Then, as shown in FIG. 32E, the second silicon layer21 is subjected to DRIE with the pattern 78 used as a mask until theetching reaches the insulation layer 160.

[0150] Then, as shown in FIG. 33A, the resist pattern 78 is removed.Then, as shown in FIG. 33B, a photoresist 79′ is formed by spraying thematerial from below. Then, as shown in FIG. 33C, the photoresist 79′ isprocessed into a resist pattern 79. Then, as shown in FIG. 33D, thesecond silicon layer 21 is subjected to DRIE with the pattern 74 used asa mask until the etching reaches the insulating layer 160. Thus, acomb-teeth electrode El contacting with the insulating layer 160 isobtained. Then, as shown in FIG. 33E, the resist pattern 79 is removed.Then, as shown in FIG. 33F, the exposed insulating layer 160 is etchedaway. At this time, the oxide film patterns 72, 74 are also removed.

[0151] According to the present invention, as described above, thetorsion bar T can be substantially completed at an earlier stage thanthe comb-teeth electrodes E1 and E2, and vice versa.

[0152] Reference is now made to FIGS. 34A-34C illustrating a micromirrorunit 100′ according to a tenth embodiment of the present invention.FIGS. 34A-34C correspond to FIGS. 2A-2C (first embodiment of the presentinvention), respectively. As described below, the micromirror unit 100′is slightly different in arrangement from the unit 100 of the firstembodiment.

[0153] Specifically, the inner frame 120 of the unit 100′ has amulti-layer structure composed of an inner frame body 121, a first innerinsulating layer 161, an inner silicon layer 170, a second innerinsulating layer 162 and electrode bases 122. The inner silicon layer170 is vertically sandwiched between the first and the second innerinsulating layers 161, 162. The outer frame 130 of the unit 100′ hasalso a multi-layer structure composed of a first outer frame member 131,a first outer insulating layer 161, an outer silicon layer 170, a secondouter insulating layer 162 and a second outer frame member 132. Theouter silicon layer 170 is vertically sandwiched between the first andsecond outer insulating layers 161, 162.

[0154] The inner frame 120 is connected to the outer frame 130 by a pairof second torsion bars 150′. Each of the second torsion bars 150′ has aninner end (on the side of the inner frame 120) and an outer end (on theside of the outer frame 130). The inner end is connected to the innersilicon layer 170 of the inner frame 120, while the outer end isconnected to the outer silicon layer 170 of the outer frame 130. Theother arrangements of the unit 100′ are similar to the counterparts ofthe unit 100. Thus, the upper and lower plan views of the unit 100′appear the same as those shown in FIGS. 1A and 1B.

[0155] Reference is now made to FIGS. 35A-35E and 36A-36E illustrating amicromirror unit fabrication method according to an eleventh embodimentof the present invention. This method employs a micro-machiningtechnique and can produce the above-described micromirror unit 100′.

[0156] In the illustrated embodiment, first an oxide film pattern 80 isformed on an SOI wafer 9, as shown in FIG. 35A. The wafer 9 has amulti-layer structure composed of a first silicon layer 22, a firstinsulating layer 161, a second silicon layer 23, a second insulatinglayer 162 and a third silicon layer 24. The second silicon layer 23 ofFIG. 35A corresponds to the inner silicon layer 170 of FIGS. 34. Thefirst silicon layer 22, the second silicon layer 23 and the thirdsilicon layer 24 may be made of a conductive silicon (or polysilicon)material doped with n-type impurities such as phosphorus or arsenic. Thefirst silicon layer 22 has a thickness of 100 μm, the second siliconlayer 23 has a thickness of 5 μm, and the third silicon layer 24 has athickness of 100 μm. Each of the first and the second insulating layers161, 162 has a thickness of 1 μm.

[0157] Then, as shown in FIG. 35B, a resist pattern 81 is formed on thefirst silicon layer 22 for masking a region to be processed into atorsion bar T. Then, as shown in FIG. 35C, the first silicon layer 22 issubjected to DRIE with the patterns 80 and 81 used as a mask untilgrooves of a predetermined depth (e.g. 5 μm) are formed. Then, as shownin FIG. 35D, the resist pattern 81 is removed. Then, the first siliconlayer 22 is subjected to DRIE with the pattern 80 used as a mask untilthe etching reaches the first insulating layer 161, as shown in FIG.35E. In the thus formed groove, a portion 22 a of the first siliconlayer 22 remains.

[0158] Then, as shown in FIG. 36A, the first insulating layer 161exposed at the step of FIG. 35E is etched away. Thus, the second siliconlayer 23 is partially exposed. The thus exposed second silicon layer 23is subjected to DRIE with the first insulating layer 161 used as a mask,as shown in FIG. 36B, until the etching reaches the second insulatinglayer 162. At this stage, the remaining portion 22 a of the layer 22 isetched away. As a result, a pre-torsion bar T′ contacting with the firstand the second insulating layers 161-162 is obtained. Then, as shown inFIG. 36C, an oxide film pattern 82 is formed on the third silicon layer24. Then, the layer 24 is subjected to DRIE with the pattern 82 used asa mask until the etching reaches the second insulating layer 162, asshown in FIG. 36D. Then, as shown in FIG. 36E, the first and the secondinsulating layers 161, 162 contacting with the pre-torsion bar T′ areremoved by immersion into etchant. At this time, the exposed oxide filmpatterns 80, 82 are also etched away. Thus, a torsion bar T is obtained.

[0159] In the above-described embodiment, at the step of FIG. 35D, theresist pattern 81 is removed, and then the layer 22 is subjected to anadditional etching step. Alternatively, the pattern 81 may remainthrough some subsequent steps to be used as a mask for making thepretorsion bar T′.

[0160] Further, in the above-described embodiment, the pretorsion bar T′in the making is held in contact with both the first and the secondinsulating layers 161-162. Thereafter, the two layers 161-162 areremoved to provide the complete pre-torsion bar T′. In this manner, thethickness of the resulting pre-torsion bar T′ is determined by thethickness of the second silicon layer 23 of the SOI wafer 9. Therefore,a precisely thin (e.g. 5 μm) torsion bar T can be obtained at thevertical center of the wafer 9.

[0161] Reference is now made to FIGS. 37A-37D and 38A-38D illustrating amicromirror unit fabrication method according to a twelfth embodiment ofthe present invention. This method again employs a micro-machiningtechnique and can produce the above-described micromirror unit 100′.

[0162] In the illustrated embodiment, first, an oxide film pattern 83 isformed on an SOI wafer 10, as shown in FIG. 37A. The wafer 10 has amulti-layer structure composed of a first silicon layer 25, a firstinsulating layer 161, a second silicon layer 26, a second insulatinglayer 162 and a third silicon layer 27. The first silicon layer 25 isformed with a groove 25 a by using the pattern 83 as a mask. The secondsilicon layer 26 corresponds to the inner silicon layer 170 shown inFIGS. 34. The first silicon layer 25, the second silicon layer 26 andthe third silicon layer 27 are made of a conductive silicon (orpolysilicon) material doped with n-type impurities such as phosphorus orarsenic. The first silicon layer 25 has a thickness of 100 μm, thesecond silicon layer 26 has a thickness of 5 μm, and the third siliconlayer 27 has a thickness of 100μm. Each of the first and the secondinsulating layers 161-162 has a thickness of 1 μm.

[0163] Then, as shown in FIG. 37B, a photoresist 84′ is formed byspraying the material from above. Then, the photoresist 84′ is processedinto a resist pattern 84, as shown in FIG. 37C. The resist pattern 84serves as a mask for a region to be processed into a torsion bar T.Then, as shown in FIG. 37D, the exposed regions of the first insulatinglayer 161 are etched away with the pattern 84 used as a mask.

[0164] Then, as shown in FIG. 38A, the resist pattern 84 is removed.Then, as shown in FIG. 38B, the second silicon layer 26 is subjected toDRIE with the first insulating layer 161 used as a mask until theetching reaches the second insulating layer 162. Thus, a pre-torsion barT′ contacting with the first and the second insulating layers 161-162 isobtained. At this stage, an oxide film pattern 85 is formed on the thirdsilicon layer 27. Then, as shown in FIG. 38C, the third silicon layer 27is subjected to DRIE with the pattern 85 used as a mask until theetching reaches the second insulating layer 162. Then, as shown in FIG.38D, the insulating layers 161-162 contacting with the pre-torsion barT′ are etched away by immersion into etchant. At this time, the oxidefilm patterns 83, 85 are also removed. Thus, a torsion bar T isobtained.

[0165] The present invention being thus described, it is obvious thatthe same may be varied in many ways. Such variations are not to beregarded as a departure from the spirit and scope of the presentinvention, and all such modifications as would be obvious to thoseskilled in the art are intended to be included within the scope of thefollowing claims.

1. A method of making a micromirror unit from a material substratehaving a multi-layer structure including a plurality of silicon layersand at least one intermediate layer, the micromirror unit including amirror forming base, a frame and a torsion bar, the method comprisingthe steps of: forming a pre-torsion bar by etching the silicon layers,the pre-torsion bar being smaller in thickness than the mirror formingbase and held in contact with the intermediate layer; and forming thetorsion bar by removing the intermediate layer contacting with thepre-torsion bar.
 2. A method of making a micromirror unit including amirror forming base, a frame and a torsion bar, the method comprisingthe steps of: preparing a first material substrate having a multilayerstructure including a first silicon layer, a second silicon layer and anintermediate layer disposed between the first and the second siliconlayers, the first silicon layer having a thickness corresponding to athickness of the torsion bar; providing a first mask pattern on'thefirst silicon layer to mask a portion of the first silicon layer that isprocessed into the torsion bar; performing first etching until the firstetching reaches the intermediate layer for forming a pre-torsion barcontacting with the intermediate layer; attaching a third silicon layerto the first silicon layer for forming a second material substrateincorporating the pre-torsion bar; subjecting the second silicon layerto second etching by using a second mask pattern arranged to avoid aregion corresponding to the pre-torsion bar, the second etching beingcontinued until the intermediate layer is exposed; subjecting the thirdsilicon layer to third etching by using a third mask pattern arranged toavoid a region corresponding to the pre-torsion bar, the third etchingbeing continued until the pre-torsion bar is exposed; and subjecting theintermediate layer exposed by the second etching to fourth etching forremoving the intermediate layer contacting with the pre-torsion bar sothat a torsion bar is obtained.
 3. The method according to claim 2,further comprising the step of forming a fourth mask pattern to mask thepre-torsion bar, wherein the fourth mask pattern forming step isperformed after the pre-torsion bar is formed in the first materialsubstrate and before the second material substrate is formed.
 4. Amethod of making a micromirror unit including a mirror forming base, aframe and a torsion bar, the method comprising the steps of: preparing afirst material substrate having a multilayer structure including a firstsilicon layer, a second silicon layer and an intermediate layer disposedbetween the first and the second silicon layers, the first silicon layerhaving a thickness corresponding to a thickness of the torsion bar;forming a first mask pattern to mask a region of the first silicon layerthat is to be processed into the torsion bar; attaching a third siliconlayer to the first silicon layer; subjecting the second silicon layer tofirst etching by using a second mask pattern arranged to avoid a regionfor forming the torsion bar, the first etching being performed until theintermediate layer is exposed; forming a pre-torsion bar contacting withthe intermediate layer by subjecting the third silicon layer to secondetching using a third mask pattern arranged to avoid a region forforming the torsion bar, the second etching being continued until thefirst mask pattern and the intermediate layer are exposed; andsubjecting the intermediate layer exposed by the first etching to thirdetching for removing the intermediate layer contacting with thepre-torsion bar so that the torsion bar is provided.
 5. A method ofmaking a micromirror unit including a mirror forming base, a frame and atorsion bar, the method comprising the steps of: preparing a materialsubstrate having a multi-layer structure including a first siliconlayer, a second silicon layer and an intermediate layer disposed betweenthe first and the second silicon layers; forming a groove in the firstsilicon layer by subjecting the first silicon layer to first etchingusing a first mask pattern arranged to avoid a region in which thetorsion bar is formed, the first etching being continued until theintermediate layer is exposed; coating the groove with a siliconmaterial; subjecting the second silicon layer to second etching using asecond mask pattern arranged to cover a region in which the torsion baris formed, the second etching being continued until the intermediatelayer is exposed; subjecting the intermediate layer exposed by thesecond etching to third etching performed on a side of the secondsilicon layer, the third etching being continued until the siliconmaterial applied at the groove is exposed; performing fourth etching ona side of the second silicon layer to remove the silicon materialexposed by the third etching, so that a pre-torsion bar made of thesilicon material and contacting with the intermediate layer is obtained;and forming the torsion bar by removing the intermediate layercontacting with the pre-torsion bar.
 6. A method of making a micromirrorunit including a mirror forming base, a frame and a torsion bar, themethod comprising the steps of: preparing a material substrate having amulti-layer structure including a first silicon layer, a second siliconlayer and an intermediate layer disposed between the first and thesecond silicon layers; forming a groove in the first silicon layer bysubjecting the first silicon layer to first etching using a first maskpattern arranged to avoid a region in which the torsion bar is formed,the first etching being performed until the intermediate layer isexposed; coating the groove with a silicon material; subjecting thesilicon material applied at the groove to second etching using a secondmask pattern to mask a region in which the torsion bar is formed, thesecond etching being continued until the intermediate layer is exposed,so that a pre-torsion bar made of the silicon material and contactingwith the intermediate layer is obtained; subjecting the third siliconlayer to third etching using a third mask pattern arranged to avoid aregion corresponding to the pre-torsion bar, the third etching beingperformed until the intermediate layer is exposed; and subjecting theintermediate layer exposed in the third etching to fourth etching forremoving the intermediate layer contacting with the pre-torsion bar, sothat the torsion bar is obtained.
 7. A method of making a micromirrorunit including a mirror forming base, a frame and a torsion bar, themethod comprising the steps of: preparing a first material substrateincluding a first silicon layer; forming a groove in the first materialsubstrate by first etching using a first mask pattern arranged to avoida region in which the torsion bar is formed; coating the groove with anintermediate layer material; applying a silicon material on the coatingof the intermediate layer material to fill the groove; forming a secondmaterial substrate having a multilayer structure which includes thefirst material substrate, an intermediate layer covering the groove ofthe first material substrate and a second silicon layer contacting withthe intermediate layer, the second material substrate incorporating apre-torsion bar that is made of the silicon material and held in contactwith the intermediate layer; subjecting the first silicon layer tosecond etching using a second mask pattern arranged to avoid a regioncorresponding to the pre-torsion bar, the second etching being performeduntil the intermediate layer material applied to the groove is exposed;subjecting the second silicon layer to third etching using a third maskpattern arranged to avoid a region corresponding to the pre-torsion bar,the third etching being continued until the intermediate layer isexposed; and subjecting the intermediate layer material exposed in thesecond etching and the intermediate layer exposed in the third etchingto fourth etching for removing the intermediate layer material and theintermediate layer to provide the torsion bar.
 8. A method of making amicromirror unit including a mirror forming base, a frame and a torsionbar, the method comprising the steps of: preparing a first materialsubstrate including a first silicon layer; subjecting the first materialsubstrate to first etching using a first mask pattern arranged to mask aregion to be processed into the torsion bar, the first etching beingcontinued until the etching reaches a depth corresponding to a thicknessof the torsion bar; preparing a second material substrate having amulti-layer structure which includes the first material substrate, anintermediate layer held in contact with an etched surface of the firstmaterial substrate and a second silicon layer held in contact with theintermediate layer; subjecting the second silicon layer to secondetching using a second mask pattern arranged to avoid a region in whichthe torsion bar is formed, the second etching being continued until theintermediate layer is exposed; subjecting the first silicon layer tothird etching using a third mask pattern arranged to avoid a region inwhich the torsion bar is formed, so that a pre-torsion bar contactingwith the intermediate layer is obtained; and subjecting the intermediatelayer exposed in the second etching to fourth etching for removing theintermediate layer contacting with the pre-torsion bar, so that thetorsion bar is provided.
 9. A method of making a micromirror unitincluding a mirror forming base, a frame and a torsion bar, the methodcomprising the steps of: preparing a material substrate having amulti-layer structure including a first silicon layer, a second siliconlayer and an intermediate layer disposed between the first and thesecond silicon layer; subjecting the first silicon layer to firstetching using a first mask pattern and a second mask pattern, the firstmask pattern being arranged to mask a region in which the torsion bar isformed, the second mask pattern being arranged to avoid a region inwhich the torsion bar is formed, the first etching being continued untilthe etching reaches a depth corresponding to a thickness of the torsionbar; removing the first mask pattern; subjecting the first silicon layerto second etching using the second mask pattern, the second etchingbeing continued until the intermediate layer is exposed, so that apre-torsion bar contacting with the intermediate layer is obtained;subjecting the second silicon layer to third etching using a third maskpattern arranged to avoid a region corresponding to the pre-torsion bar,the third etching being continued until the intermediate layer isexposed; subjecting the intermediate layer exposed in the third etchingto fourth etching for removing the intermediate layer contacting withthe pre-torsion bar to provide the torsion bar.
 10. A method of making amicromirror unit including a mirror forming base, a frame and a torsionbar, the method comprising the steps of: preparing a material substratehaving a multi-layer structure which includes a first silicon layer, asecond silicon layer having a thickness corresponding to a thickness ofthe torsion bar, a third silicon layer, a first intermediate layerbetween the first and the second silicon layers, and a secondintermediate layer between the second and the third silicon layers;subjecting the first silicon layer to first etching using a first maskpattern and a second mask pattern, the first mask pattern being arrangedto mask a region in which the torsion bar is formed, the second maskpattern being arranged to avoid a region in which the torsion bar isformed, the first etching being continued until the etching reaches adepth corresponding to a thickness of the torsion bar; removing thefirst mask pattern; subjecting the first silicon layer to second etchingusing the second mask pattern, the second etching being continued untilthe first intermediate layer is exposed, so that a third mask pattern isformed at the first silicon layer disposed on the first intermediatelayer; subjecting the first intermediate layer exposed in the secondetching to third etching using the third mask pattern, the third etchingbeing continued until the second silicon layer is exposed, so that afourth mask pattern is formed at the first intermediate layer disposedon the second silicon layer; subjecting the second silicon layer exposedin the third etching to fourth etching using the fourth mask pattern,the fourth etching being continued until the second intermediate layeris exposed, so that a pretorsion bar is formed between the first and thesecond intermediate layers; subjecting the third silicon layer to fifthetching using a fifth mask pattern arranged to avoid a regioncorresponding to the pre-torsion bar, the fifth etching being continueduntil the second intermediate layer is exposed; and performing sixthetching to the second intermediate layer exposed in the fifth etchingand to the first intermediate layer on the pre-torsion bar, so that thefirst and the second intermediate layers contacting with the pre-torsionbar are removed to provide the torsion bar.
 11. The method according toclaim 9 or 10, wherein the second etching forms a comb-teeth electrodeat one of the mirror forming base and the frame.
 12. The methodaccording to claim 9 or 10, wherein an etching process other than thesecond etching forms a comb-teeth electrode at one of the mirror formingbase and the frame.
 13. A method of making a micromirror unit includinga mirror forming base, frames and a torsion bar, the method comprisingthe steps of: preparing a material substrate having a multi-layerstructure which includes a first silicon layer, a second silicon layerhaving a thickness corresponding to a thickness of the torsion bar, athird silicon layer, a first intermediate layer between the first andthe second silicon layers, and a second intermediate layer between thesecond and the third silicon layers; subjecting the first silicon layerto first etching using a first mask pattern arranged to avoid a regionin which the torsion bar is formed, the first etching being continueduntil the first intermediate layer is exposed; forming a second maskpattern on the first intermediate layer exposed in the first etching,the second mask pattern being arranged to mask a region in which thetorsion bar is formed; subjecting the first intermediate layer to secondetching using the second mask pattern, the second etching beingcontinued until the second silicon layer is exposed; removing the firstmask pattern; subjecting the second silicon layer to third etching usingthe first intermediate layer exposed by the removal of the first maskpattern, the third etching being continued until the second intermediatelayer is exposed, so that a pre-torsion bar contacting with the firstand the second intermediate layers is obtained; subjecting the thirdsilicon layer to fourth etching using a third mask pattern arranged toavoid a region corresponding to the pre-torsion bar, the fourth etchingbeing continued until the second intermediate layer is exposed; andperforming fifth etching to the second intermediate layer exposed in thefourth etching and to the first intermediate layer on the pre-torsionbar, so that the first and the second intermediate layers contactingwith the pre-torsion bar are removed to provide the torsion bar.
 14. Themethod according to claim 13, wherein the etching performed to eachsilicon layer is inductively coupled plasma etching.
 15. The methodaccording to claim 14, wherein the frames include a first frame and asecond frame connected to the first frame by the torsion bar.
 16. Amicromirror unit comprising: a mirror forming base; frames having amulti-layer structure including a plurality of silicon layers and atleast one intermediate layer; and a torsion bar which is smaller inthickness than the mirror forming base, the torsion bar defining an axisabout which the mirror forming base is rotated relative to the frames,the torsion bar having an end connected to a portion of the siliconlayers that is held in contact with the intermediate layer.
 17. Themicromirror unit according to claim 16, wherein the frames include afirst frame and a second frame each of which includes a silicon layerand an intermediate layer held in contact with the silicon layer, thetorsion bar having one end connected to the silicon layer of the firstframe and another end connected to the silicon layer of the secondframe.
 18. The micromirror unit according to claim 16, wherein theframes include two intermediate layers between which only one of thesilicon layers is disposed, the torsion bar being connected to said oneof the silicon layers.
 19. The micromirror unit according to claim 18,wherein the frames include first and second frames each of whichincludes two intermediate layers between which only one silicon layer isdisposed, the torsion bar having one end connected to said one siliconlayer of the first frame and another end connected to said one siliconlayer of the second frame.
 20. The micromirror unit according to claim16, wherein the intermediate layer is made of an insulating material.21. The micromirror unit according to claim 16, wherein the mirrorforming base is provided with a first comb-teeth electrode, the framesbeing provided with a second comb-teeth electrode cooperating with thefirst comb-teeth electrode to move the mirror forming base.
 22. Themicromirror unit according to claim 17, wherein the first frame isprovided with a third comb-teeth electrode, the second frame beingprovided with a fourth comb-teeth electrode cooperating with the thirdcomb-teeth electrode to move the first frame and the mirror formingbase.